A process for producing unsaturated aldehydes and/or unsaturated acids from gas-phase olefins using a catalyst is a typical example of catalytic vapor phase oxidation. Typically, the catalytic vapor phase oxidation may be exemplified by a process for producing (meth)acrolein and/or (meth)acrylic acid by oxidizing propylene or propane, or a process for producing (meth)acrolein and/or (meth)acrylic acid by oxidizing isobutylene, t-butylalcohol or methyl-t-butylether.
To perform the catalytic vapor phase oxidation, solid-phase multi metal oxides are generally used as a catalyst. A composite oxide containing molybdenum and bismuth or molybdenum and vanadium, or a mixture thereof is generally used as a catalyst in the process for producing (meth)acrolein and/or (meth)acrylic acid. At least one type of catalyst in the form of granule is packed into a reaction tube, at least one reactant and air containing molecular oxygen as an oxidizing agent, molecular nitrogen as an inert gas, or raw material containing water vapor is in contact with the catalyst in the reaction tubes to perform vapor phase oxidation.
Generally, propane, propylene, isobutylene, t-butylalcohol or methyl-t-butylether is subjected to two-step catalytic vapor phase oxidation to form (meth)acrylic acid as a final product. More particularly, in the first-step reaction zone, propylene or the like is oxidized by oxygen, diluted inert gas, water vapor and an optional amount of catalyst to form (meth)acrolein as a main product. In the second-step reaction zone, (meth)acrolein obtained from the preceding step is oxidized by oxygen, diluted inert gas, water vapor and an optional amount of catalyst to form (meth)acrylic acid.
The catalyst used in the first-step reaction zone is an oxidation catalyst based on molybdenum-bismuth (Mo—Bi), which oxidizes propylene or the like to form (meth)acrolein as a main product. Additionally, a part of (meth)acrolein is further oxidized on the same catalyst to form (meth)acrylic acid partially. The catalyst used in the second-step reaction zone is an oxidation catalyst based on molybdenum-vanadium (Mo—V), which oxidizes (meth)acrolein-containing mixed gas produced in the first-step reaction zone, particularly (meth)acrolein, to form (meth)acrylic acid as a main product.
Since the catalytic vapor phase oxidation for producing (meth)acrolein and/or (meth)acrylic acid is performed at high temperature (200˜600° C.) and is a highly exothermic reaction, heat transfer fluid such as molten salt is provided on the outer surface of reaction tubes to remove heat of reaction, whereby reaction temperature in the reaction tube is maintained at a predetermined temperature.
Reactors for carrying out the above process are realized in such a manner that each of the above two steps are implemented in one system or in two different systems. Further, the first-step reaction zone for producing (meth)acrolein by oxidizing propylene or the like may be divided into two or more reaction zones, as described in Korean Patent No. 10-0450234.
In the process for producing unsaturated aldehyde and/or unsaturated fatty acid from gas-phase propane, propylene, isobutylene, t-butylalcohol or methyl-t-butylether (referred to as ‘propylene or the like’ hereinafter), abnormal behaviors may occur at the initial start-up due to high reactivity of propylene or the like. Such abnormal behaviors deteriorating the catalyst include excessive rise of temperature of hot spots in catalyst layers due to explosive reaction according to material composition, excessive reaction due to high reaction temperature, or excessive rise of temperature of hot spots in catalyst layers, which is generated by high heat of reaction.
Further, in the case of producing (meth)acrolein and/or (meth)acrylic acid by feeding propylene or the like at a high space velocity and high concentration, the load of conversion in the catalyst layer is increased, resulting in excessive rise of temperature of hot spots in the catalyst layers. Therefore, heat accumulation occurs in the vicinities of the hot spots, which causes various problems such as loss of active ingredients from the catalyst layer and reduction in the number of active sites caused by the sintering of metal components, resulting in degradation in the quality of the catalyst layer.